In general, there are three methods of obtaining fingerprint information from human beings, namely, indirect method, optical method and non-optical method. The indirect method involves the use of scanners or cameras for scanning or photographing fingerprints printed on a paper, and the non-optical method utilizes the principle of electric field.
The most efficient and common method is the optical method. As shown in FIG. 1, an apparatus for optically inputting fingerprints includes a light source (1) for illuminating light on a fingerprint region, a diffusing plate (2) for diffusing light generated from the light source (1), a prism (3), a lens (4) for forming a fingerprint image to be formed on a fingerprint capturing surface (3-a) of the prism (3), and an image sensor (5) for converting the fingerprint image formed on the lens (4) to an electrical signal.
In the optical fingerprint input apparatus thus described, when the fingerprint region touches the fingerprint capturing surface (3-a) of the prism (3), light beams from the light source (1) are reflected from and absorbed in the ridge lines and valleys, where the reflected light beams are incident on the prism (3), images of which are formed on the lens (4), and the light beams having their images formed on the lens (4) are incident on the image sensor (5) to allow the images of the fingerprint to be inputted.
However, there is a drawback in the optical fingerprint input apparatus thus described according to the prior art in that miniaturization of the apparatus is restricted by the thickness of the lens, making it difficult to be integrated with other apparatus.
In order to cope with the above-mentioned drawback, a prism is thinned to form a thin plate. However there arises another drawback in that an angle of an inclined surface becomes larger as the thickness of prism is thinned to proportionally produce a worsened trapezoidal distortion of the fingerprint images, making it impossible to be applied to mobile apparatus where a thin-light-miniaturized look is valued high as in cellular phones.
There is still another drawback in that oil from a human's hand or face can stain a fingerprint contact region of an optical fingerprint input apparatus when the apparatus is applied to a cellular phone, making it difficult to distinguish between oil information and fingerprint information and to verify fingerprints.